A conventional permanent magnet motor will be described referring to Japanese Laid-Open Patent Publication 119175/1987. The permanent magnet motor (henceforth referred to as “surface magnet motor”) such as this has a rotor provided with a plurality of permanent magnets fixed to the periphery of a rotary shaft, spacers between the permanent magnets, and pressing members formed of non-magnetic material for pressing radially on the surface of the permanent magnets.
In terms of this surface magnet motor, since the magnets are fixed to the surface of the rotor, the surface magnet motor has excellent torque linearity characteristics in terms of linearity between torque and current. Thus, under high load, large torque can be obtained with small current, providing excellent controllability.
However, the foregoing surface magnet motor requires the pressing members and associated components for fixing the permanent magnets, resulting in a complicated structure.
In order to solve the above mentioned problems, Japanese Laid-Open Patent Publication 197292/2000 discloses an interior permanent magnet motor.
In terms of the technology disclosed in the patent publication, the interior permanent magnet motor has rectangular, axially penetrating slots for the magnets, provided circumferentially in a rotor iron core at equal intervals in an amount for the number of poles of the rotor, with the long side of the slots oriented circumferentially along the iron core and the short sides oriented radially along the iron core, a permanent magnet being fixed in each of the magnet slots so that, with the magnetic-pole faces diametrically oriented in the iron core, neighboring magnetic poles are opposites, wherein the outer peripheral shape of the magnetic-pole portions of the rotor, formed along the magnetic-pole faces on the outer peripheral side of each of the permanent magnets, is such that each of the rotor magnetic-pole portions is arcuate so that, in the circumferentially central portion the distance from the iron core center is longest, and in the portions between poles the distance from the iron core center is shortest.
In terms of the interior permanent magnet motor, the permanent magnets are embedded in the rotor iron core, so that the pressing members for fixing the permanent magnets are unnecessary.
However, in the interior permanent magnet in the above described Japanese Laid-Open Patent Publication 197292/2000, the thickness of the outer portion of the rotor iron core is not prescribed, so that a problem has been that the cogging torque—the torque variation that when no current is flowing occurs due to harmonic components of the air-gap magnetic flux density—becomes somewhat large, and the torque linearity cannot be improved.
The present invention is intended to solve the above described problems and has as an object the provision of a permanent magnet motor in which the cogging torque is lowered and the torque linearity is improved.